I was also looking for a MPG tracker and checked a couple of apps. Either expensive or not very helpful. Finally I found one app called MileageMeter, which I use now to keep track of the fuel consumption of my and my wifes car and I am pretty happy with it. It shows the MPG for every refill, works with multiple cars and you can get some nice statistics like a MPG graph and gas cost per mile etc. Price tag is USD 3.95, but they have a free demo version at MileageMeter - Track your car's gas consumption on your Blackberry
